By now you have probably read about some problems this lens has with canon's system, most likely because of the 1.6x crop, (widely discussed on dpreview's forums) but what I'm about to tell should give you more insight of how bad this problem can and may be.
You should be warned that this isn't gonna be a pretty review, but rest assured, I'm no troll trying to diss Tamron. I'll still give credit where credit is due.
Another warning: most of what I'll describe only happens with Canon's dSLRs (1.6x) and even so, not all lenses suffer from these problems. As you'll see later, this lens - when working right - can rival with ANY lens in this range image and sharpness wise, and for a fraction of the price... you just have to be lucky or persistant enough.
It all started when, in April, I received the lens and started right away to notice some inconsistency in focus/out-of-focus pictures, for no aparent reason. Then I noticed that sometimes I could see objects on focus closer to me than the one that should be on focus. It was clear what the problem could be, so I ran a few home-made tests with objects at different distances and with focus test charts to see if it wasn't really my fault.
It wasn't. The lens suffered from severe front-focus from f2.8 to f8 and from 28 to 50mm. For instance, if the shot was taken at 28mm/f2.8, specially some meters afar, it would show severe front focus... I'd say a good 5cm or so. If it was shot at 75mm/2.8, it would be tack sharp! I've used an EF 70-200/2.8L IS USM and several Canon primes so I know what *sharp* can look like, and I could tell you that when this boy is sharp, it will rival with ANY zoom lens in this range, regardless of price.
So, I contacted Tamron UK because their Service Dept. was famous on dpreview's forums for taking care of this somehow already infamous situation and I learned that I would have to send the lens thru my national importer (lens bought elsewhere within the EU) so I didn't have to pay expenses but that it was a known issue and it should be solved without a problem. Then with my national importer I learned that it would be sent to Tamron Germany.
One month later I had no news about the lens so I started getting a bit uneasy and had to really insist with them to give me news of some sort, or
at least a point of situation. That was when I received an email saying that the lens behaviour was erroneus because of bad use from the owner and that I would have to pay 163€ for the repair. Mind you, this is a 315€ lens with a couple of weeks we're talking about.
At this time I was plain p|ssed and I had to be a bit rough and direct in my argumentation. It had happened before with another Portuguese guy I know, with the same lens model but they were immediatey sorry and took care of it... with me the situation was getting a bit uglier.
After two or three emails they said they were sorry and that they would take care of the lens immediately with no expenses what so ever.
After two weeks I received the lens and immediately carried the same tests I did before sending the lens to callibration and the results were absolutely the same! I even doubted they had even done anything to it!
By the time I sent the next email, the person I was taking care of the problem with was on vacations so it took about one week to get some feedback. When I did receive a new email, from another technician who would take care of this process until the end, they asked if I could send the camera along with the lens so they could, this time, adjust the lens by 1:1 specifically for my camera. I thought "what the hell... if I do have to wait for it, I may as well send it all along so that I can receive it all working at 100%" because honestly, I did want this lens to work flawlessly as this is the range I work on mostly and as this lens is the smaller and lighter and cheaper in its class, it was a no-brainer.
As I had a couple of concerts to shoot at the the time I had to delay it by a couple of weeks and I seized the oportunity to send along my Tamron 17-35/2.8-4 so they could do some tweaking to get rid of some extra softness I was experiencing in the right side of the frame, which, by the way, they handled pretty fine and the lens is flawless now.
So I sent the lens free of charge by UPS and the next day (about 12h later!) it was in Germany already (hooray for UPS) and one week later I had it all back in my hands. Needless to say I immediately went to test it. Because I still noticed quite a lot of inconsistency in "real world" tests I decided to make a more analysable and thorough test. On a side note, the zoom ring also felt "harder" to handle after this last "intervention".
This time the lens came back even worse than before... the behaviour would be completely erratic and the lens wouldn't perform correctly even above 50mm like it used to!
At this point it was obvious there was no hope for this lens and I could only hope I could still return it after four months of "non use". My contact on Tamron said it himself that I should return the lens because even though I could send it again I should not be hopeful to receive it any better than this. He also said that I could quote him as authorization for the store to receive my lens back. Mind you, by E.U.'s laws, I only have a 15-day period to send a given item back for full refund, no questions asked, and this was more than four months later already. If Tamron would've told me to send the lens back, back when the problem started, that's what I'd have done.
The store said there was no problem and the process started. They would have to receive the lens, send the lens to Tamron, Tamron would pay them and finally, they would pay me. I had already decided to use the money to get a Canon EF 50/1.4 USM for a number of reasons I'll write about of later.
Through the process the store people told me 4 different times and in different contexts I would receive the money but it all changed when they already had Tamron's money with them and they decided to send me yet another Tamron 28-75/2.8. For as p|ssed as I could be, there wasn't anything I could do thousands of kilometers away from them but Karma is a bitch so I hope they'll pay in another way. On a side note, they are photo-palme.de, so I urge you not to buy there anything unless you like to sponsor a-holes and take risks.
When the lens arrived and as I had to sell it, I decided to test it to get sample shots to show its state and I was surprised when I found out this lens was working great. Goes to show that you _can_ get a great 28-75/2.8. The focus was still nothing close to Canons 24-70/2.8L but sharpness was right up there without a doubt. The Canon still holds a bit more natural saturation and contrast but nothing reeeeally noticeable or in other words, not something worth the 3 more times it costs compared to the Tamron alone. Canon's offer is worth it for the whole package: construction (no comparison here), focus performance (idem) and then the bit better color and contrast. Maybe you can even had to the equation the extra wide range if that's your cup of tea. Sure it weighs a ton compared to Tamron but you can notice where it is going to.
Anyway, I ended up replacing the 28-75/2.8 for an EF 50/1.4 USM because:
1) No other 3rd party brand has any zoom in this range that doesn't have its quirks and that can offer me peace of mind regarding this usual reverse engineering problems. Sigma's 24-70/2.8 and 28-70/2.8 have problems, Tokina's 28-70 also has problems. So I'm all for no compromising as of now.
2) Still not rich enough to afford Canon's L range, though I loved the 24-70/2.8L USM.
3) It's faster than any zoom available (obviously) and I can use that in concerts; it's sharper (idem); offers a good focal distance for venues and finally, it goes for about the same amount of money.
Overall Rating: 6 » You may or may not get a good copy. If you do, it's a 9. If you don't, it's a 2 or 3. I figured I would toss a number somewhere in between.
Build Quality Rating: 7 » You get what you pay for, or more or less. It does feel fragile but not to the point where it'll desintegrate in your hands. Just don't take it to the mosh pit. OTOH, it pays as a lightweight so it's perfect for walkaround use.
Price Rating: 10 » Provided you get a good copy, it's a great bang for the buck.